Dental alloy and use therefore

ABSTRACT

The invention relates to a dental alloy consisting of 20 to 35% chromium, 0 to 10% molybdenum, 0 to 3% manganese, 0.5 to 2% tin, 2 to 10% gallium, 2 to 8% indium, 0 to 8% silicon, 1.5 to 15% tungsten, 0.1 to 2% aluminum, cobalt as remainder, and impurities, and to the use of the dental alloy for the machining production of dental superstructures, in particular crowns, bridges and combined dental prostheses.

The invention concerns a dental alloy for dentistry a well as the use of this dental alloy for the machining or metal cutting manufacture of dental superstructures, in particular crowns, bridges and combined dental prostheses on prepared pillars and implants.

Dental superstructures, that is, dental prostheses elements, are, in contrast to dental implants, elements of dentistry which are exclusively seated externally upon correspondingly prepared teeth or suitable receiving elements (abutments) of dental implants. According to the state of the art, these dental superstructures are produced by taking imprints from patients and then casting using a wax model in a lost wax process. The lost wax process is very laborious due in particular to the high manual labor component. Beyond this, the multiple shape copying allows introduction of errors, so that measurement tolerances of 100 μm are rarely satisfied.

From EP 1 173 136 B1 of the present applicant a suitable thermal alloy for the lost wax process is known, which comprises, besides cobalt, in balance up to 0.4% carbon, 0.1 to 1.0% manganese, 0.1 to 1% silicon, 20 to 30% chromium, up to 1% nickel, 3 to 7% molybdenum, up to 0.75% iron, 0.1 to 5% gold and 0.1 to 4% platinum. A further embodiment this dental thermal alloy also contains gallium and/or indium in a proportion of 0.1 to 16%. These alloys are specifically formulated for the lost wax process.

Further, from KaVo Dental GmbH, 88400 Biberach, a ceramic material for dental superstructures is known, in particular full crowns, under the name “KaVo Everest BIO HPC-Blank” which is essentially a zirconium silicate ceramic. This material is shaped to the desired shape basically by machining or cutting and is subsequently sintered at approximately 1,500° C. With this type of ceramic crown, and also with those of zirconium dioxide and aluminum oxide (Al₂ O₃), there is the disadvantage, that these materials have a very low breaking elongation of nearly 0. Due to this high stiffness it occurs that during chewing movements high load-peaks are transmitted via the artificial structure, essentially un-cushioned, directly to the jaw bone of the patient. As a consequence, the sharp fibers, which separate the tooth in the clinical crown and the anatomic crown and which are responsible in part for the durable anchoring of the tooth in the jaw bone, are as strongly and in certain cases over-stressed.

It is further a disadvantage that in the machining or cutting processing of these ceramics, wall thicknesses of at least 0.6 mm must remain intact. Accordingly, for preparing the tooth to receive this crown manufactured of ceramic, the tooth must be milled down by at least this wall thickness plus tolerance and cement layer, whereby in certain cases an influencing on the dental pulp and a therewith associated sensitivity can result for the patients. Accordingly, the indications for combined prosthetic care, in particular in the case of a firmly bonded tooth replacement, is strongly limited.

It is further disadvantageous, that the ceramic is worked essentially in an incomplete “half sintered” condition. For the final development of the shape a subsequent centering process is thus necessary. In the thermal stressing due to the subsequent or follow-up sintering at, for example, 1,500° C., shrinkages can result, which may interfere with the fitting precision of the dental replacement. Therewith the technical employability of this ceramic is limited.

Notwithstanding the above, the cutting or machining form of manufacture essentially offers a high dimensional accuracy. Further, the ceramic material is very economical and the fabrication is simpler than in the case of the lost wax casting technique. Thus, there is a need in dentistry for an automated manufacture for tooth replacement elements. Until now however metallic materials in the dental art have not been suitable for a machining or cutting manufacture.

Due to the substantial production advantages in a machining manufacture of dental superstructures from both cost aspects as well as maintaining dimensional trueness, it is the task of the invention to provide a suitable material for machining manufacture and for tooth replacement, which does not exhibit, or at least minimizes, the above-mentioned disadvantages.

In accordance with the invention this task is solved with a dental alloy which is based upon a noble metal free alloy.

In the following description of the invention, reference to percent is on the basis of weight of the respective components relative to the total alloy.

The material composition is based on the universal alloy similar to the dental sinter alloys known's from EP 1 173 136 B1. Surprisingly, by addition of tungsten and aluminum, very good machining characteristics can be achieved, wherein the remaining positive characteristics of the known alloys remain essentially intact. In contrast to known dental sinter alloys it became possible to completely omit the elements platinum and gold, thereby the material costs are reduced.

The inventive dental alloy is characterized, besides a very good machining characteristic, also thereby, that the material can be processed with a CNC process to a wall thickness of 0.2 mm. The amount of ablation or erosion of a tooth to be fitted with a crown due to the necessary material removal is thus cut in half in comparison to the known ceramic materials to be machined. Here it was observed, that more delicate cervical column contact of crowns serve cosmetic and esthetically. They are however in particular effective in combating resorption of gums and prevent collection of materials in the furrows or depression. The resulting gaps to the clinical tooth at the preparation border is sealed against bacteria with cement or adhesive materials. They are not rinsed when following wearing (secondary cavities).

With the inventive alloy, the execution of any useful construction idea, for which there is a prosthetic demand, can be carried out. The hardness, machinability and the ductile elasticity are in harmony. The finest constructions can be machined from blanks of this alloy with a precision of ≦20 μm by machining, in particular with a five access CNC-machine without unduly wearing out the tool.

In the processing or machining, by the use of highly pure starting materials, and by adjustment of a very extreme fine crystal alloy, high ductile elastic alloys can be obtained, which can be easily processed by machining and besides this can be highly polished. Polished surfaces prevent attachment and build-up of plaque.

It is further advantageous that blanks produced from this alloy can be machine processed with extremely short (round) shavings or turnings. This behavior is the basis for the machinability of the alloy with NC/CNC-milling machines. The prosthetic part produced from the blank has therewith a final configuration.

The inventive NMF-alloy (noble metal free alloy) forms an alloy almost like modeling clay with characteristics excellently adapted for the intended use. In the melt a pudding-like consistency forms, which is characterized by rounded corners and edges in the blanks obtained from the molds. Particularly to be noted is that, on solidification of the melt, only an extremely low volumetric contraction of approximately 1.8% can be detected. This extremely low volumetric contraction should also be responsible therefore, that the solidified material occurs without defect in the blanks. Therewith the blank or perform in the shape of the cast block provided for the machining processing is practically defect-free, as could be confirmed by taking sections or slices and x-ray examination.

It has further been determined experimentally, that the inventive alloy exhibits a very low oxidation rate which is very important for patient health. In the so-called “7-day-test” the rate of oxidation is clearly below the standard value of 100 μg per 7 days.

It can further be mentioned, that the inventive dental alloy may contain, in addition to the components listed in the claims, also traces of impurities, such as for example those present in starting materials or those added in the course of the manufacturing.

The element tungsten, which is apparently important for the very good machinability, should be present in a proportion of 2.5 to 10% in the preferred embodiment of the dental alloy.

Preferably molybdenum is contained in a weight percentage of 3 to 8% and/or manganese in a proportion of 0.1 to 1% and/or silicon in a proportion of 0.2 to 4% in the alloy.

By the inventive addition of metals of the rare earth type, namely, cerium and/or yttrium, as well as the further elements tantalum, niobium and/or zirconium, an increase in the adhesion or bonding strength of the present dental alloy in the metal-ceramic composite or compound is achieved. Likewise, the proportion of 0.5 to 2% tin contributes to the improvement of the adhesion between the present dental alloy and the ceramic material to be bonded to.

The inventive alloys can additionally include conventional suitable flux materials (adhesion promoting materials). The inventor is aware of a particularly suitable binder or adhesion promoter as disclosed in DE 100 225 59 B4.

The alloys are suitable for manufacture of various dental superstructures, such as for example crowns, bridges, combined tooth replacements, etc. They are also excellent for manufacture of dental superstructures with various coatings or layers such as for example feldspar ceramics and/or hydrothermal nano leucite ceramics.

One particular advantage of the inventive alloy is its ease of workability in the manufacture of the cast semi-finished part and a very good machining processability. In the production of the melt in the blank mold no scoriae form, and due to the oxide reduction by immunization of the mold (seed formers) an adhesion of the melt to the mold is prevented.

The physical and chemical characteristics can be maintained in comparison to known NMF-dental sinter alloys. The mechanical characteristics allow for the first time the machining with NC/CNC-machines. In comparison to ceramic materials, the inventive dental alloy is characterized, depending upon the cast process, with a breaking elongation of up to 20%.

Further, no allergic reactions are known for the inventive alloys.

One general problem for the manufacture of metal-ceramic-compounds is the, as a rule, differential thermal coefficients of expansion relationships of the metal alloys on the one hand and the ceramic materials on the other hand, as a consequence of which cracks or spalling could occur. It is desired however to have a bonding between the metal and the ceramic of almost unlimited duration, which is not disturbed by cracks or spalling in the ceramic.

It has been found that the thermal coefficient of expansion of the inventive alloy can easily be adjusted to correspond to the thermal coefficient of expansion of the conventionally employed high melting ceramics, which lies in the vicinity of 13.9×10⁻⁶ (at 500° C.; as set forth in DIN EN ISO 9693).

According to a particularly preferred embodiment, the inventive alloy thus exhibits a thermal coefficient of expansion which corresponds or is similar to the thermal coefficient of expansion of the employed ceramic material. Preferably the inventive alloy has a thermal coefficient of expansion of 13.9×10⁻⁶ to 15.0×10⁻⁶, in particular approximately 14.7×10⁻⁶ (at 500° C.).

Overall the inventive composition achieves an exclusion of microcavities and vacuum inclusions in the blanks produced from the melt or sintering, as has been shown by x-ray examination. The liquid pudding-like melt in addition exhibits an excellent mold filling quality. Preferably the melt is introduced into the mold in accordance with the so-called “roll-over process”. Thereby a fine dendritic crystal structure results. The rapid solidification of the melt prevents formation of contraction cavities. Even in the original melt there was no cavity. Therewith a high economic efficiency of the melt process is to be expected. 

1. A dental alloy comprising: 20 to 35% chrome, 0 to 10% molybdenum, 0 to 3% manganese, 0.5 to 2% tin, 2 to 10% gallium, 2 to 8% indium, 0 to 8% silicon, 1.5 to 15% tungsten, 0.1 to 2% aluminum, and the balance cobalt as well as impurities.
 2. The dental alloy according to claim 1, characterized by a proportion of 3 to 8% molybdenum.
 3. The dental alloy according to claim 1, characterized by a proportion of 0.1 to 1% manganese.
 4. The dental alloy according to claim 1, characterized by a proportion of 0.2 to 4% silicon.
 5. The dental alloy according to claim 1, characterized by a proportion of 2.5 to 10% tungsten.
 6. The dental alloy according to claim 1, characterized by a proportion of 5 to 8% gallium.
 7. The dental alloy according to claim 1, additionally containing at least one element selected from tantalum, niobium, cerium, yttrium and zirconium with a total proportion of 0.1 to 3%.
 8. The dental alloy according to claim 1, wherein the alloy has a thermal coefficient of expansion of 13.9×10⁻⁶ to 15.0×10⁻⁶ (at 500° C.).
 9. A method for manufacture of dental superstructures comprising: forming a blank of an alloy comprising: 20 to 35% chrome, 0 to 10% molybdenum, 0 to 3% manganese, 0.5 to 2% tin, 2 to 10% gallium, 2 to 8% indium, 0 to 8% silicon, 1.5 to 15% tungsten, 0.1 to 2% aluminum, and the balance cobalt as well as impurities, determining the shape of a dental receiving surface upon which the dental superstructure is to be seated, and machining the blank to form a surface complementary to the dental receiving surface upon which the superstructure is to be seated.
 10. The method as in claim 10, wherein said machining is NC/CNC machining.
 11. The method as in claim 10, wherein said superstructure is a crown, bridge or combined tooth replacement. 